1. Technical Field
The present invention relates to a method for manufacturing a differential device and a differential device.
2. Related Art
A vehicle is provided with a differential device such as a front differential device or a rear differential device that, during turning etc., distributes the driving force outputted from the engine between the corresponding left and right driving wheels. As another aspect of the differential device, there is also a center differential device that distributes the driving force outputted from the engine between the front and rear driving wheels. The differential device includes a ring gear to which driving force is transmitted and a differential case that is joined to the ring gear and rotates fixedly with the ring gear. A joining method by welding is employed as a method for joining the ring gear and the differential case together. For example, beam welding is performed on the contact position between a ring gear made of carbon steel and a differential case cast using cast iron, and thereby the ring gear and the differential case are joined together.
Technologies for improving the strength of welding between the ring gear and the differential case are proposed. For example, Japanese Unexamined Patent Application Publication No. 2013-18035 discloses a differential device in which a gap is formed between the gear-side welding surface of a ring gear and the case-side welding surface of a differential case to allow the heat distortion of the ring gear to be absorbed and allow the residual stress of the welding bead to be reduced. The differential device further has a hollow through which welding gas generated during laser welding is discharged, and can thereby suppress the occurrence of weld defects.
Further, WO 2011/089706 discloses a welded structure of a differential device that has a penetration hole penetrating such that one end is open to a hollow for causing beam welding to function as penetration welding and the other end is open to the outer surface of a differential case, and that causes the air in the hollow that has expanded during beam welding to flow out to the external space and can thereby prevent weld defects. Further, WO 2012/143986 discloses a differential device that has an escape surface on an annular flange located on the inner peripheral side with respect to an annular contact site that is formed by one side surface of an annular flange of a differential case and a side surface of a flange of a ring gear, and that can suppress weld defects by releasing welding gas.
Here, in the case where the ring gear and the differential case are joined together by welding, cracking may occur in the differential case. The main factor of the cracking is that, during welding, a brittle structure is produced due to the dissolving-out of carbon contained in the cast iron. The vicinity of a gate through which molten cast iron is poured during casting is lower in cooling rate than other areas, and has a higher ratio of the pearlite structure having a relatively high carbon equivalent. Hence, an area of the differential case formed near the gate during casting is more likely to experience cracking during welding than other areas. In particular, in the case where the differential case and the ring gear are welded together over the entire periphery, when an overlap where the base point and the end point of welding overlap is located in the area formed near the gate, the amount of heat input to the area with a high pearlite ratio is increased, and structure embrittlement is expanded and cracking is likely to occur.